Method of making tin plate resistant to oxidation



Se t. 24, 1963 i w. E. BOGGS 3,105,022

METHOD OF MAKING TIN PLATE RESISTANT T0 OXIDATION Filed April 5, 1962 REMEL rnva FRE- OXIDATION @QR (ABOVE 450 (FROM :00 7'0 425 F1 ELECTROLYTIC ELEC momv PLA ruvs aEPos/r/o/v or ALLOY/N6 ELEMENT INVENTOR WILL IAM E. B0665 Affo n ey of the tin surface.

Patented Sept. 24, 1963 3,105,022 METHOD OF MAKING TIN PLATE RESISTANT T OXIDATION William E. Boggs, Pitcairn, Pa., assignor to United States Steel Corporation, a corporation of New Jersey Filed Apr. 5, 1962, Ser. No. 185,470 1 Claim. (Cl. 204-37) This invention relates to the manufacture of tin plate and, in particular, to a method of making a product highly resistant to oxidation.

This is a continuation-in-part of my application Serial No. 68,381, filed November 10, 1960, now abandoned.

Conventional tin plate discolors when stored for extended periods under certain conditions and this gives rise to a major problem of long standing in the tin-plate industry. If the discoloration is severe, the product may be rejected by the customer on the basis of appearance, regardless of the corrosion resistance of the material. The presence of a thick oxide film on tin plate, furthermore, may adversely affect the solderability and lacquerability of the material. Such oxidation is especially serious in the humid storage of tightly packed sheets of tin plate prior to can manufacture.

The object of my invention, therefore, is to provide tin plate which will not discolor when stored for long periods of time.

Chemical and electrochemical passivation treatments which form complex protective phosphate or chromate films on tin surfaces are commonly used to inhibit the formation of tin oxide on tin plate and to enhance the corrosion resistance of the material when fabricated into cans. Tin plate treated by such methods, however, may etch unevenly when exposed to certain food products, such as condensed milk and green beans, and furthermore, quite often exhibits poor lacquer adherence.

It is known that the addition to tin of up to 0.1% by weight of zinc, phosphorus or indium, tends to prevent the formation of colored films on the tin surface when heated. I have discovered that not only zinc but a small amount of aluminum, chromium, magnesium, calcium, gallium and germanium, if added to the tin layer deposited on a low-carbon steel sheet to make tin plate, limits the rate of formation of oxide on the surface to a fraction of the rate at which it forms on pure tin. My theory to explain this result is that minute amounts of the alloying or impurity elements in solid solution in the tin are preferentially oxidized at all of the microscopic sites where nucleation and growth of a tin-oxide film normally occurs.

The rapid formation of minute crystal-lites of the oxides of the alloying or impurity elements effectively blocks or precludes the formation of any tin oxide, even under highly oxidizing conditions, for very long periods of time. Even after very long times, when tin oxide does finally begin to form, it forms more slowly than is normally the case in tin that does not contain these alloying elements. The amount of oxide of the alloying elements so formed is negligible; it cannot be clearly and positively detected by sensitive microbalance weighing techniques, but only by electron-microscope examination Thus, the addition of small amounts of these alloying elements improves the over-all resistance of the tin to oxidation without impairing its quality or appearance.

The alloying elements are preferably applied to electrolytically coated tin plate before the latter is heated to a temperature above the melting point of tin, i.e., the

usual remelting or flow-brightening step to which electrolytic tin plate is customarily subjected. The material should then be given a short heating or pre-oxidation at a temperature below the melting point of tin, to oxidize the alloying element although this treatment is not essential in the case of zinc. This, according to my theory, inhibits subsequent oxidation of the tin by atmospheric oxygen. It also causes the alloying element to diffuse to sites of preferred oxidation.

A complete understanding of the invention may be obtained from the following detailed description and explanation which refers to the accompanying drawing illustrating the present preferred embodiment. The single FIGURE of the drawing is a diagrammatic showing of the apparatus which I employ for carrying out the method of my invention.

Referring now in detail to the drawing, low-carbon steel strip 10 is passed through an electroyltic tinning line 11 of known type, wherein a coating of tin is deposited thereon amounting to, say 0.25 or 0.50 pound per base box. The strip next passes through a cathodic electroplating tank 12 containing, for example, a solution in ethanolamine (anhydrous) of the chloride of the desired alloying element such as aluminum. By this means, an addition of from 0.001 to 0.5% of aluminum by weight of the tin coating is added to the latter. This addition is preferably about 0.25%.

It is important that the strip leaving tank 12 have a uniform film of electrolyte thereon and, to this end, it should be passed between one or more pairs of squeegee rollers. This insures uniform brightening of the tin coating during the subsequent melting stage.

The strip next traverses a melting tower 13 wherein it is heated by known means to a temperature above the melting point of tin. The tin coating is thereby fused temporarily and brightened. The strip emerging from tower 13 is cooled by contact with the atmosphere to a temperature below the melting point of tin and then passes immediately into a pre-oxidation tower 14 where it is maintained at a temperature of from 300 to 425 F., under oxidizing conditions, preferably about 400, for a short period, i.e., from 2 to 10 minutes, e.g., 4 minutes, preferably no longer than necessary but at least long enough to oxidize the aluminum addition. This causes the alloying addition to diffuse throughout the tin as previously stated, to points of preferred oxidation, and the formation of crystallites of the oxide of the alloying element, without oxidizing the tin.

The alloying additions of the following elements (column 1) in the amounts indicated (column 2) effected a substantial reduction (column 3) in the amount of oxida tion of the tin coating compared to pure tin under the same conditions, viz., prolonged exposure to oxidation at about 400 F.

Table 1 Percent Amount by Reduction Element Added Weight of in Oxidation Tin, percent of Alloy Compared to Pure Tin 0. 023 82 0.21 100 0.059 89 0.061 9]. Chromium 0. 044 83 Calcium 0. 034 85 7ino 0.055 92 The percentage ranges of additions of the several elements, effective for the purpose of the invention are as follows:

Table II 7 Percent Aluminum 0.001 to 0.5 Magnesium 0.0005 to 0.5 Chromium .006 to 0.5 Calcium 0.006 to 0.5 Gallium 0.006 to 2.5 Germanium 0.05 to 1.0 Zinc 0.006 to 2.5

amount of alloying element solid-solubility limit in tin since, otherwise, eutectic formation would occur which would impair the integrity of the tin coating and increase the oxidation rate of the tin.

Magnesium and calcium, like aluminum, may be electro-deposited from anhydrous ethanolamine solutions of their salts. Germanium mus-t be applied by the known vapor-deposition process and this method may also be used for the other elements.

It is evident from the foregoing that the invention has the important advantage that it makes possible the production of electrolytic tin plate with a greatly reduced tendency to discolor on oxidation. The method of my invention, furthermore, is simple, inexpensive and efiec tive.

Although I have disclosed herein the preferred embodiment of my invention, I intend to cover as well any change or modification therein which may be made with out departing from the spirit and scope of the invention.

I claim:

A method of making tin plate which comprises the steps of:

(a) electrolytically depositing a coating ofrtin on a base of low-carbon sheet steel,

([1) depositing over said coating from 0.0005 to 2.5% by Weight of the tin deposit, of an alloying element selected from the group consisting of aluminum, calcium, chromium, gallium, germanium, magnesium and zinc,

(c) heating the base to a temperature above the melting point of tin thereby melting the tin coating,

(d) cooling the coated base to a temperature below the melting point of tin,

(e) and thereafter maintaining the coated base at a temperature of from 300 to 425 F. for from 2 to 10 minutes under oxidizing conditions, thereby oxidizing said alloying element and forming crystallites of the oxide thereof effective to oxidation of the tin coating.

Nachtman Mar. 25, 1947 Allen Oct. 4, 1955 FOREIGN PATENTS Great Britain Great Britain May 9, 1938 May 11, 1955 inhibit subsequent 

